Epitaxy at very high temperatures, such as silicon carbide (SiC) epitaxy which is conducted at around 1600° C., is very difficult to accomplish due to the erosion of the hot zone parts of the reactor and thermal losses due to radiation. The hot zone materials are usually made from coated graphite surrounded by thick insulation consisting of rigid graphite insulation or graphite felt insulation. The insulation generally increases the time necessary to heat up and cool down the system.
FIG. 1 is a section view of a portion of a prior art reactor showing a rectangular susceptor 102 surrounded by insulation 104. A quartz tube 106 surrounds rectangular susceptor 102. Quartz tube 106 is surrounded by heating coil 108. It is also suitable, and sometimes preferably to use a cylindrical susceptor within a cylindrical coil. Insulation 104 covers the front and back of susceptor 102 (not fully shown here). Due to the large distance from the susceptor to the coil and the large amount of insulation, this system is long in heat-up and cool-down time.
Susceptor 102 is heated by rf heating via coil 108 surrounding cylindrical quartz tube 106. Quartz tube 106 is usually not water cooled. This is an unfortunate choice of design as the distance to the coil is closest where the side walls (not shown) are. In order to improve the coupling of the coil to the graphite susceptor, the rectangular blocks are made thicker which increases the heat up and cool down time. Furthermore, the insulation is very thick on the top and bottom of the susceptor which also increases the cool down time. Finally, the temperature uniformity is not very good in such a configuration and the required power is high.
The insulation consists of rigid graphite insulation or graphite felt. Both the rigid insulation and to a lesser extent the graphite felt insulation are somewhat conducting, which adds to the heating losses in case of rf heating. Ideally, these materials should be avoided in the hot zone. Furthermore, the insulation being fairly thick will increase the distance between the graphite to the coil, which reduces the coupling efficiency.
Due to the thermal insulation and the fact that the susceptor is far away from the coil the heat up and specifically cool down takes a very long time, which dramatically reduces the throughput of the reactor. The heat up can be improved by moving the graphite susceptor closer to the coil and by increasing the power. In the ideal case the object that is to be heated should be close to the rf coil and no insulation should be used. This is generally not an acceptable design because of the heat losses and power necessary.
FIG. 2 depicts another prior art reactor section having a cylindrical coil 202 with a susceptor 204. Two half moon shaped shells 206, 208 form a split cylinder. The two half moon shells 206, 208 are separated by two side walls 210, 212, thus forming an interior chamber 214 through which the process gases flow. It is noted that the term “cylinder” as used herein is not limited to a hollow object with a circular cross sectional shape, but includes hollow objects having other cross-sectional shapes.
In any of these arrangements the rf heating into the susceptor will cause hot spots due to arcing between the sides and the top and bottom walls. The arcing will erode the graphite and damage the susceptor. Arcing may be avoided if the side walls are electrically isolated from the top and bottom using e.g. high purity silicon carbide. This, however, can add expense to the reactor design.
With chlorinated chemistry it is possible to grow the epitaxial layers very quickly. The throughput, however, is limited by the heat up, cool down, pump down, and reload times. The heat up can today take about 40 min to 1 hour. Cool down takes 2-3 hours. Pump down and reload times are generally quick and depend much on the system and the person running it. Therefore, there is a need for a system that will allow a very quick heat up and cool down cycle.